1. Field of the Invention
The invention relates to a printed circuit board and fabrication thereof, and more particularly relates to gold fingers of a printed circuit board and method for forming related products.
2. Description of the Related Art
Printed circuit boards (PCB) are currently in wide use in various electronic equipments. With progress of technology, wiring density of printed circuit boards is more and more condensed. It is important for printed circuit boards to improve wiring density and while not sacrificing process stability, reliability, low cost and yield.
In the process of fabricating gold fingers of a printed circuit board, a nickel and gold layer is required to be deposited on a wire pattern in the wire bonding substrate to provide stable electrical connection for the wire bonding substrate and chips.
Generally, prior art requires plating bus lines, extending from circuits to the periphery of the wire bonding substrate, for providing a conductive-line path when plating nickel and gold on the wire bonding substrate to form a nickel and gold layer on the area without covering a solder mask. However, the plated buses must use some area of the wire bonding substrate and affects wiring density of the substrate.
Further, as shown in FIG. 1, when the wiring density of the bonding substrate 108 is increased, the line pitch P of the gold fingers 102 is decreased. The effective bonding top width W and the bottom gap S of gold fingers 102 are main specifications of a wire bonding substrate 108. The bonding width W is required to large enough, and limited to a sufficient bottom gap S in a limited line pitch P. However, when fabricating a gold finger 102, the nickel and gold layer 106 covers the top and sidewall of the copper pad 104, and when thickness of the nickel and gold layer 106 increases, the line pitch P is decreased. Thus, shot or open, easily occurs and increase of the bonding width W is limited. Further, when the bottom gap S is too small, migration issues may occur.